Voltage controlled oscillators (VCO) are well known in the art. The operating frequency of VCO's is often controlled by the application of a voltage to a tuning varactor incorporated in their designs. The operating frequency is then modulated by an information signal using a modulation varactor also incorporated in their design. When the frequency of a VCO is changed by applying a different voltage to the tuning varactor, the tuning varactor capacitance changes and the percentage of the total tank capacitance comprised by the modulation varactor changes. Since the modulating voltage delivered to the modulation varactor is constant and the capacitance percentage changes, the frequency deviation is not constant when the operating frequency of the VCO changes. A technique to compensate for this changing modulation sensitivity would be to de-couple the modulation varactor from the tank as the operating frequency is increased. This would accomplish flat modulation sensitivity over operation frequency which is much desired in today's transmitters.
However, the compensation to achieve flat modulation sensitivity produces a harmonic interference problem. The problem occurs when a harmonic of the VCO tracks through a secondary resonance of the VCO. The resonance allows tighter coupling from the top of the tank to the modulation varactor at the frequency of resonance. As the VCO is tuned and a harmonic reaches this resonant frequency, the harmonic is coupled tighter to the modulation varactor than it was outside of the resonance. The instantaneous voltage across the varactor is changed with the addition of a larger harmonic voltage and this shifts the quiescent operating point of the varactor. Since the capacitance vs. voltage characteristic of the varactor is nonlinear, a constant modulation voltage now produces a different capacitive variation and hence a different frequency deviation. The graph of FIG. 1 shows this phenomenon. The horizontal axis in the graph represents the VCO operating frequency. The vertical axis represents frequency deviation or modulation sensitivity of the VCO. The dotted line 102 shows a typical modulation sensitivity variation as an oscillator harmonic tracks through a secondary resonance in the VCO tank. The harmonic energy is coupled tighter to the modulation varactor than it was outside of the resonance. This harmonic-resonance interaction shifts the bias point of the varactor and changes the modulation sensitivity and the result is a deviation response that is not flat with respect to the operating frequency. The solid graph 104 shows a desired modulation sensitivity as the frequency of the VCO varies.
The approach VCO designers take to deal with this problem is to identify the resonance that is causing the deviation problem and move it out of band. Although this technique works, the VCO is still exposed to other resonances that might move in band due to component and process variations. Since almost every structure of the VCO substrate will resonate somewhere, it is unlikely that the designer will be able to keep all resonances out of all harmonic bands across circuit build variations. This leaves VCOs that use this modulation compensation technique at risk to this deviation flatness problem over process variations. It is therefore clear that a need exists for a VCO that would be tolerant to harmonics-resonance interactions and other interferences.